1. Field of the Invention
The present invention relates to a circuit design apparatus, a backup power supply, a circuit design program, and a capacitor energy amount calculation method.
2. Description of the Related Art
Memories each including a backup power supply have been known. For example, Japanese Unexamined Patent Application Publication No. 2014-63209 discloses a flash memory including a backup power supply including a capacitor.
In recent years, there have been advances in the reduction in size and profile of flash memories. The reduction in profile of a backup power supply in such a flash memory is therefore needed.
As a backup power supply in a flash memory, for example, a conducting polymer capacitor or an electric double-layer capacitor (EDLC) is known. However, it may be difficult for these capacitors to flexibly adapt to the reduction in profile, because they have a configuration in which a small number of thick layers are covered with a resin package.
In the case of multi-layer ceramic capacitors (MLCCs), the reduction in profile is relatively easily achieved by reducing a large number of thin ceramic layers therein. However, it is known that some of MLCCs used as capacitors in backup power supplies have characteristics in which a capacitance significantly changes in accordance with an applied voltage, a voltage application period, or a temperature.
Energy supplied by a capacitor is calculated using equation (1) where C represents a capacitance, V1 represents a discharge start voltage, and V2 represents a discharge end voltage.The amount of energy=1/2·C·(V12−V22)  (1)
A capacitor energy amount calculation method in the related art represented by equation (1) is based on the assumption that the capacitance of a capacitor is substantially constant. In a case where the amount of energy of a capacitor such as an MLCC whose capacitance varies in accordance with a usage environment is calculated using the capacitor energy amount calculation method in the related art, a calculated amount of energy may be different from an actual amount of energy. For example, in a case where the number of MLCCs required as a backup power supply is calculated, the amount of energy of the MLCCs is calculated under the assumption that these MLCCs have the lowest capacitance in a usage environment. As a result, the calculated required number of MLCCs may exceed the actual required number of MLCCs. The increase in the required number of MLCCs makes it difficult to reduce the profile of a backup power supply including the MLCCs because of a limited footprint.